Using Molecular Scenes to Predict the Net Direction of an Acid-Base Reaction
Problem Given that 0.10 M HX (blue and green) has a pH of 2.88, and 0.10 M HY (blue and orange) has a pH of 3.52, which scene (Fig 18.6) best represents the final mixture after equimolar solutions of HX and Y^− are mixed?
Plan A stronger acid and base yield a weaker acid and base, so we have to determine the relative acid strengths of HX and HY in order to choose the correct molecular scene. The concentrations of the acid solutions are equal, so we can pick the stronger acid directly from the pH values of the two acid solutions. Because the stronger acid reacts to a greater extent, fewer molecules of it will be in the scene than molecules of the weaker acid.
Solution The HX solution has a lower pH (2.88) than the HY solution (3.52), so we know right away that HX is the stronger acid and X^− is the weaker base and that HY is the weaker acid and Y^− is the stronger base. Therefore, the reaction of HX and Y^− has K_c > 1, which means the equilibrium mixture will have more HY than HX. Scene 1 has equal numbers of HX and HY, which would occur if the acids were of equal strength, and scene 2 shows fewer HY than HX, which would occur if HY were stronger. Therefore, only scene 3 is consistent with the relative acid strengths.